From: Joerg Roedel <jroedel@xxxxxxx>
When PTI is enabled on x86-32 the kernel uses the GDT mapped
in the fixmap for the simple reason that this address is
also mapped for user-space.
The efi_call_phys_prolog()/efi_call_phys_epilog() wrappers
change the GDT to call EFI runtime services and switch back
to the kernel GDT when they return. But the switch-back uses
the writable GDT, not the fixmap GDT.
When that happened and and the CPU returns to user-space it
switches to the user %cr3 and tries to restore user segment
registers. This fails because the writable GDT is not mapped
in the user page-table, and without a GDT the fault handlers
also can't be launched. The result is a triple fault and
reboot of the machine.
Fix that by restoring the GDT back to the fixmap GDT which
is also mapped in the user page-table.
Fixes: 7757d607c6b3 x86/pti: ('Allow CONFIG_PAGE_TABLE_ISOLATION for x86_32')
Reported-by: Guenter Roeck <linux@xxxxxxxxxxxx>
Tested-by: Guenter Roeck <linux@xxxxxxxxxxxx>
Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
---
arch/x86/platform/efi/efi_32.c | 8 ++------
1 file changed, 2 insertions(+), 6 deletions(-)
diff --git a/arch/x86/platform/efi/efi_32.c b/arch/x86/platform/efi/efi_32.c
index 324b9332..05ca142 100644
--- a/arch/x86/platform/efi/efi_32.c
+++ b/arch/x86/platform/efi/efi_32.c
@@ -85,14 +85,10 @@ pgd_t * __init efi_call_phys_prolog(void)
void __init efi_call_phys_epilog(pgd_t *save_pgd)
{
- struct desc_ptr gdt_descr;
-
- gdt_descr.address = (unsigned long)get_cpu_gdt_rw(0);
- gdt_descr.size = GDT_SIZE - 1;
- load_gdt(&gdt_descr);
-
load_cr3(save_pgd);
__flush_tlb_all();
+
+ load_fixmap_gdt(0);
}
void __init efi_runtime_update_mappings(void)
--
2.7.4
